It is known to provide acoustic liners in fan and other casings to reduce noise generated by the engine during operation. Typically they comprise acoustically resistive layers (i.e. layers which filter out particular wavelengths) above honeycomb cavities. Typical configurations consist of one or more resistive layers, backed respectively by one or more honeycomb cavities. Those with one resistive layer and one honeycomb layer are known as single layer liners, while those with two honeycomb layers separated by an acoustically resistive septum layer are known as double layer liners. These liners are tuned to attenuate noise within a range of frequencies.
Modern engines may produce noise in more than one distinct range of frequencies. For example core engine noise (i.e. due to combustion) may be in the range of about 200 Hz-630 Hz), and turbine noise may be in the range of 4000 Hz to 6300 Hz. To overcome this, liners tuned to different frequencies are provided at different locations downstream of the source of the noise. Hence one has the choice of either providing a substantial amount of the gas flow path with an acoustic liner in order to attenuate noise to a required level, and thus suffering a weight penalty due to the increased number of liner panels making up the liner and/or increased duct length to accommodate the acoustic panels, or employing few liner panels to keep weight low and thus risking an insufficient amount of attenuation.
Hence an acoustic liner panel which can attenuate noise over two or more distinct frequency ranges whilst being relatively compact is highly desirable.